A refractive optical element or a diffractive optical element has been used for an optical system to be mounted on, for example, a camera or a liquid crystal projector in recent years. The use of multiple refractive optical elements or multiple diffractive optical elements different from each other in refractive index, wavelength dispersibility, and secondary dispersibility enables each of optical systems that reduce chromatic aberration to be realized by using a small number of optical elements. A glass material (i.e., inorganic material) has been conventionally used in an optical element of which each of those optical systems is formed. However, an optical element composed of a glass material has a heavy weight, and is apt to splinter. Accordingly, investigation into an optical element composed of a synthetic resin material (e.g., plastic material) which has a light weight and which is available at a low cost is in progress.
A synthetic resin has a hardness lower than that of glass. Accordingly, the resin has excellent processability and a high degree of freedom in its shape. Glass is processed by pulverization, grinding, or compression molding at high temperature. In contrast, the synthetic resin is easily processed by injection molding or cast molding in a short time period. Accordingly, a cost for the processing of the synthetic resin is much lower than a cost for the processing of glass.
However, a much larger number of restrictions are imposed on the optical properties of a synthetic resin for an optical element which has been currently put into practical use such as a refractive index, wavelength dispersibility, and secondary dispersibility than on the optical properties of glass. Accordingly, optical design similar to that of an optical system composed of a glass material is not established, so it is impossible to reduce the number of optical elements, or it is difficult to correct required chromatic aberration.
In addition, an optical element composed of a synthetic resin material is superior to an optical element composed of a glass material in processability, but is apt to be flawed. Accordingly, the number of fine flaws on the surface of the optical element increases with the passage of time, so the light transmittance of the optical element reduces over time.
In addition, the elastic modulus, coefficient of water absorption, coefficient of thermal expansion, and temperature dependence of the refractive index of a synthetic resin are extremely large as compared to those of glass. For example, the coefficient of water absorption of polymethyl methacrylate (PMMA) is about 2.0%, which is an extremely large value. Accordingly, the volume of PMMA largely fluctuates in accordance with a temperature change, so a focal length changes. Accordingly, it has been difficult to use a synthetic resin in applications where high optical stability is required.
The inferiority of an optical element composed of a synthetic resin material to an optical element composed of a glass material is summarized as follows:
(1) the range of each of optical properties such as a refractive index, wavelength dispersibility, and secondary dispersibility is narrow;
(2) the former optical element has a low elastic modulus and a low surface hardness, so its surface is apt to be flawed;
(3) the former optical element has a high coefficient of thermal expansion (about 10−4/° C.), so its shape largely changes owing to heat, and it is poor in optical stability;
(4) the temperature dependence of the refractive index of the former optical element is large, so the former optical element is poor in optical stability; and
(5) the former optical element has a high coefficient of water absorption, so it is poor in optical stability.
In view of the foregoing, proposals have been made to solve the above-mentioned problems and to expand the range in which an optical element formed of a synthetic resin material can be used. That is, there has been proposed a composite material obtained by mixing a synthetic resin with an inorganic material, the composite material having a higher elastic modulus, a lower coefficient of water absorption, a lower coefficient of thermal expansion, and a smaller temperature dependence of a refractive index than those of a conventional synthetic resin. There has been also proposed an optical composite having good wavelength dispersibility of a refractive index, good secondary dispersibility, and a good light-scattering rate.
Japanese Patent Application Laid-Open No. 2005-162902 discloses an optical composite material which is made highly elastic by: adding metal oxide fine particles of silica, alumina, or the like to a cyclic olefin-based graft copolymer; and crosslinking the resultant.
Japanese Patent Application Laid-Open No. 2003-213067 discloses an optical composite material whose coefficient of thermal expansion is reduced to 8×10−5/° C. or less by dispersing silica fine particles in an acrylic resin.
Japanese Patent Application Laid-Open No. 2004-83669 discloses an optical composite whose coefficient of water absorption is reduced by: hydrolyzing a metal aliphatic acrylalkoxide to form an acrylate in which an extremely minute granular metal oxide is dispersed; and polymerizing the acrylate.
Japanese Patent Application Laid-Open No. 2003-73558 discloses an optical composite material obtained by dispersing ultra-fine particles each composed of a metal oxide in a polymer (PMMA). The document discloses that, in this case, the ultra-fine particles and the polymer form a nanocomposite, so it is possible to realize the mechanical, thermal, and optical effects of the polymer such as an elastic modulus, a heat forming temperature, gas barrier property, a glass transition point, and a crystallization temperature which can have not been conventionally realized. It should be noted that the term “nanocomposite” generally refers to a composite material obtained by imparting a function that cannot be exerted by any particle but a particle having a nano-size such as a quantum confining effect to a composite composition with a polymer.
Japanese Patent Application Laid-Open No. 2004-269773 discloses a method of forming a nanocomposite involving uniformly dispersing a metal oxide having polar groups (mainly a hydroxyl group) on its surface in a thermoplastic resin (PMMA). The document discloses that a composite having such a high surface hardness that the composite can resist abrasion with a wiper or the like is produced by the method.
The optical composite material described in Japanese Patent Application Laid-Open No. 2005-162902 described above is produced by adding the fine particles of a metal oxide such as silica having an average particle size of 50 nm or less to a cyclic olefin-based graft copolymer having a polysiloxane structure in any one of its side chains. In this case, however, the fine particles of the metal oxide necessarily aggregate owing to micro phase separation peculiar to a sol-gel reaction. Therefore, the optical composite material is applied to an optical element in a state where a domain is enlarged, so light scattering occurs, and it is difficult to obtain desired optical performance.
In addition, in Japanese Patent Application Laid-Open No. 2003-213067, an optical composite material having a low coefficient of thermal expansion is obtained by directly mixing an acrylic resin with 30 to 90 wt % of colloidal silica fine particles having an average particle size of 10 to 20 nm. However, when the addition amount of the silica fine particles each serving as an inorganic component is 30 wt % or more, the silica fine particles necessarily aggregate. As a result, light scattering occurs and a transmittance reduces, so it is difficult to obtain desired optical performance.
In addition, the optical composite material described in Japanese Patent Application Laid-Open No. 2004-83669 obtains the inorganic polymer of a metal alkoxide in the presence of an acrylate monomer in which minute granules are dispersed. However, when the acrylate monomer is polymerized to increase a molecular weight, compatibility between an inorganic polymer component and an organic resin component is bad, so a micro phase-separated structure is produced. As a result, the minute granules aggregate. When the optical composite material is applied to an optical element in a state where the domain of the micro phase-separated structure is enlarged, light scattering occurs, and a transmittance reduces.
In addition, Japanese Patent Application Laid-Open No. 2003-73558 describes that mechanical physical properties are improved by a nanocomposite effect. However, investigation has revealed that the optical composite material disclosed in the specification has improved mechanical physical properties, but does not deviate from a value range that can be predicted on the basis of a rule of mixtures. The term “value range that can be predicted on the basis of a rule of mixtures” refers to a value range obtained by summing the volume fractions of the physical properties of the metal oxide ultra-fine particles and the polymer (PMMA), and is called an additivity range. That is, the mere modification of the surface of each of the metal oxide ultra-fine particles with an acidic group does not allow deviation from the additivity range, though the modification improves the dispersibility of the metal oxide ultra-fine particles in the polymer.
In addition, Japanese Patent Application Laid-Open No. 2004-269773 describes that the mechanical physical properties of an optical composite material such as a bending strength, a bending modulus, and a coefficient of linear expansion are improved by a nanocomposite effect. However, this case does not allow deviation from an additivity range any more than Japanese Patent Application Laid-Open No. 2003-73558 described above. That is, the mere provision of the surfaces of metal oxide ultra-fine particles with polar groups (mainly a hydroxyl group) does not allow deviation from the additivity range, though the provision improves the dispersibility of the metal oxide ultra-fine particles in a polymer. A nanocomposite effect taking an additivity range into consideration will be described in detail in the section titled <Nanocomposite effect> in BEST MODES FOR CARRYING OUT THE INVENTION described below.